[llvm-dev] RFC: Killing undef and spreading poison

[Friedman, Eli via llvm-dev]

On 10/18/2016 5:06 AM, Nuno Lopes via llvm-dev wrote:
> Another use is, for example, to implement bit-fields:
> a.x = 2
> becomes:
> %v = load %a
> %v2 = freeze %v  ; %v could be uninitialized data (poison)
> %v3 = ... bitmasking...
> store %a, %v3

It seems like you're saying that an integer load which touches any 
uninitialized byte of memory results in poison.  Therefore, load %a 
simplifies to "poison", and your proposed lowering of a bitfield write 
throws away the value of every other adjacent bitfield.  Am I missing 
something?

-Eli

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